Hypertrophy is an important mechanism by which the heart adapts to various kinds of stress such as pressure and volume overload. The process of cardiac hypertrophy produces characteristic structural changes. Since the passive electrical properties of heart muscle are strongly determined by its morphology, the spread of electrical current will depend on the structure of the preparation. Accordingly the changes in cellular morphology seen in myocardial hypertrophy would be expected to alter the distribution of the electrical impulse. Therefore, the objective of the proposed study will be: (1) to define the electrical properties of hypertrophied cardiac muscle; (2) to correlate electrical properties with the ultrastructural changes of hypertrophy; (3) to determine if the electrical equivalent circuit of hypertrophied cardiac muscle fibers differs from that of normal muscle; (4) to determine if an alteration in the relation between electrical excitation and mechanical contraction is responsible for the impaired contractility of hypertrophied muscle. The following methods will be used to achieve the objectives of the proposal: (1) analysis of the voltage-time course of the transmembrane action potential; (2) cable analysis of the passive voltage response of muscle fibers to applied current; (3) analysis of the frequency dependence of the membrane capacity; (4) investigation of the relation between contractile response assessed by optical methods and the voltage-time course of the membrane potential controlled by a modified single-gap voltage clamp technique. Cardiac hypertrophy offers a unique opportunity to study interrelations among structural alterations and electrical and contractile function. By determing how hypertrophy alters muscle physiology, it may be possible to gain further insight into the mechanisms of excitation and contraction in both normal and hypertrophied cardiac muscle.